Pebble and cobble transport on a steep, mega-tidal, mixed sand and gravel beach

Abstract

Natural tracer pebbles and cobbles were deployed on a steep (slope: 0.1) mixed sand and gravel mega-tidal beach located at the head of the Bay of Fundy. Two experiments were carried out to investigate the response of natural pebble- and cobble-sized material, both to energetic wave conditions during individual storm events, and to tidal-only forcing conditions between storms. Net displacement of different tracer sizes was the focus of the first experiment, while the effect of different shapes was tested in the second. The pebbles and cobbles were painted in fluorescent colors, labeled, and their positions measured at low tide over periods of 10–12days. The tracers were emplaced at mid-tide level on the beach face at low water, and later recovered and re-deployed on successive low tides. Net travel distance and direction were measured upon recovery. Hydrodynamic forcing conditions and local morphology at mid-tide level on the beach face were monitored continuously during the experiments. In the first experiment, recovery rates were 100% during calm and moderate wave conditions (Hs≤ 0.5m), but decreased significantly in more energetic wave forcing (Hs up to 1m). Recovery rates were lower for smaller tracers. During a three day wind-wave event, net displacements of up to 48m were measured. Comparable net displacements were observed for all tracers. Maximum displacements were 39m onshore, 42m offshore, and 46m alongshore. In calm conditions, maximum displacements were less than 1m, indicating that the bottom stresses due to tidal currents alone are too low to cause significant tracer displacement on time scales of weeks, despite the high tidal range. In the second experiment, recovery rates were lower, due to more energetic wave forcing: significant wave heights reached Hs >0.5m on 80% of the particle tracking days and Hs >1m on 30% of the days. Flat pebbles and cobbles reached the longest transport distances, with a maximum net displacement of 98m. The flat tracers were observed to be flipped by the shore break and to “ride” the swash bore, and were in this manner transported long distances, shoreward up to 66m. In both experiments, the directions of alongshore displacements were consistent with deviations of the angle of wave incidence from the shorenormal. Tracer burial and re-exposure was observed – via rotary imaging sonar – with the evolution and migration of m-wavelength ripples. The study allowed a direct correlation of pebble and cobble tracer displacements to hydrodynamic forcing during individual storm events, with consideration of tracer particle size, and shape.